Vasudeva Prakash left his job as a mechanic in Hyderabad three years ago for what he calls a more lucrative career: taking part in clinical trials on generic drugs.

For two years, Prakash participated in trials of drugs being tested to treat HIV/AIDS and other diseases for contract research organizations (CROs) hired by global pharmaceutical companies. The drugs tested at Indian CROs have been key in getting several hundred medicines approved for sale around the world.

Yet, Prakash did not follow international guidelines for testing – and the CROs that hired him didn’t require him to. He says that to earn more money he would participate in back-to-back trials on different drugs with gaps of only a few weeks or even a few days, instead of waiting the 90 days that the World Health Organization recommends.

Half of more than a dozen volunteers interviewed by Reuters across four cities – Chennai, Hyderabad, Bengaluru, and New Delhi – also said they waited much less than 90 days between trials. In the past three-to-four years, they said they spent several months at a time in different cities so that they could participate in as many studies as possible.